This invention relates to the manufacture of integrated-circuit wafers and more particularly, to a method and an apparatus for inspecting lithographic masks and reticles used in the fabrication of such wafers and/or for inspecting the wafers themselves.
Feature widths on masks and reticles used in making integrated circuits have continued to shrink. Thus, for example, masks for one-to-one scanning printers may contain features less than about two micrometers (.mu.m) in width, reticles for one-to-one step-and-repeat cameras may contain features less than about one .mu.m and X-ray masks may contain feature widths of only about 0.5 .mu.m.
Ideally, workpieces such as masks, reticles and wafers should be inspected for defects down to about half the minimum feature size. In practice, though, this is often not possible. For example, one commercially available mask inspection system is capable of detecting a minimum-size defect of only about one .mu.m, and this capability degrades to about two .mu.m in complex areas of the mask under inspection. (Hereinafter, the term "mask" is to be construed to mean either a mask or a reticle. This invention is applicable to the inspection of either type of element. And, as will be specified later below, the invention is also applicable to the inspection of wafers. But, for illustrative purposes, the discussion both in this background section and in the detailed description below will be primarily directed to inspecting masks.)
Conventional systems accomplish mask inspection by bright-field illumination of a portion of a chip pattern on the mask. A signal derived therefrom is compared with another signal representative of a corresponding portion of another supposedly identical chip pattern. The second portion may be viewed on-line, as described in "An Automated Mask Inspection System-AMIS," by J. H. Bruning, M. Feldman, T. S. Kinsel, E. R. Sittig and R. L. Townsend, IEEE Transactions on Electron Devices, Vol. ED-22, No. 7, July 1975, pp. 487-495, or as embodied in the KLA-101 Automatic Photomask Inspection System made by KLA Instruments Corporation, Santa Clara, Calif. Alternatively, a comparison signal representative of the second portion may be stored. Or such a comparison signal may be derived from software (as in the KLARIS system made by KLA). In any case, differences between the two signals are detected as defects.
In practice, the minimum-size defect that can be detected by an inspection system of the aforespecified type is set by false error indications arising from misalignment between the chip patterns being compared. At a given misalignment, the minimum detectable defect is defined as the defect which produces a signal as large as the false signal arising from the misalignment.
Misalignment between the chip patterns may arise from residual alignment errors in the inspection systems, mismatched optical distortions, mask distortions, linewidth variations, etc. Although software algorithms can help to compensate for misalignment, they are limited both by data quantization errors and by the fact there may not actually be a perfect alignment condition.
Accordingly, workers in the field have directed efforts at trying to devise improved inspection systems of the type specified above. In particular, their goal was a system capable of detecting defects smaller than those detectable by known systems. It was recognized that such efforts, if successful, could contribute significantly to improving the overall process of fabricating integrated circuits.